Inorganic nitrates



Bfiilifi Patented July 31, 1962 fire 3,047,440 INQRGANIC NITRATES Jesse E. Hughes, Bridgeport, and Leroy E. Layton, Raymond Lozier, and Edmond J. Nolan, Woodhury Heights, N.J., assignors to E. I. du Pont de Nemours and Company, Wilmington, Bah, a corporation of Delaware No Drawing. Filed .l'uly 19, E56, tier. No. 598,968 6 Claims. (Cl. 149-8) invention relates to Waterproof pellets of inorganic nitrates. More particularly, this invention relates to inorganic nitrate pellets adapted for use directly in largediameter boreholes as the main explosive charge.

Because of the considerably lower cost of explosive composition tree of explosive nitric esters and nitrocornpounds and because of the decreased hazards connected with their handling and storage, explosive compositions consisting essentially of inorganic nitrates and nonexplosive fuel sensitizers have gained wide acceptance in blasting operations utilizing large diameter (4 to 10 inch) boreholes-i Much of the economy of the agents in lost, however, because of the water-solubility of the inorganic nitrate salts. Inasmuch as a large proportion of the boreholes contain substantial amounts of water, either from seepage in the terrain or from surface water from rain, the nitrate salt explosive compositions are packaged in watertight containers, usually metal. The containers represent a substantial proportion of the total cost of the charge, although they contribute very little, if any, to the energy of the charge. Of even greater consequence, however, is the reduction in the amount of charge which can be loaded into a borehole. The borehole walls are irregular in contour, vary in diameter, and contain numerous cracks and crevices. The maximum diameter of the com tainer which is introduced into the borehole cannot exceed the minimum diameter of the borehole. Inasmuch as the containers are rigid and cannot be tamped to conform to the contour of the borehole without sacrificing fluid-tightness, much of the volume of the borehole is unoccupied. The lack of full utilization of the borehole volume requires the drilling and charging of more boreholes or larger boreholes than would otherwise be necessary to accomplish the desired blasting effect. Recent work on the development of flexible synthetic film-forming polymeric materials as packaging material for such compositions shows promise, however, the packaging material is costly and preserving a water-tight seal despite rough field handling, especially during lowering into the borehole past the jagged projections in the walls, is diflicult. If a sufiicient thickness of film to insure against tearing and puncture is used, much of the flexibility is lost and again full utilization of the borehole cannot be achieved.

Much work has also been done in the past to provide the inorganic nitrate salts with a coating of water-proofing material. Crystals and granules of the salts have been mixed, sprayed, tumbled, glazed and otherwise treated with petroleum derivatives, Waxes, natural and synthetic resins, starch and cellulose derivatives, and many other materials. At best, only slight improvement in water resistance was obtained, not sufiicient to permit using the salts directly in a wet borehole.

Accordingly, an object of the present invention is to provide inorganic nitrate salts in such form that they can be used directly in the borehole tor blasting. A further objcct is to provide inorganic nitrate salts in such form that they are completely resistant to water, even after prolonged immersion. Additional objects will become apparent as this invention is more fully described.

We have found that the attempts to waterproof incrganic nitrate salts failed in the past primarily because (1) the coating lacked uniformity to such degree that openings remained for the entrance of water, (2) the amount of coating used was insufiicient to completely encase the granules of salt, (3) coated particles stuck together during hardening of the coating, and, upon subsequent separation, openings were left in the coatings, (4) the coatings were cracked by changes in density of the salt particles, particularly ammonium nitrate, due to temperature changes, and (5 the coating material was not sufficiently impervious to water in the thickness of the layer about the salt particle. We have further found that the foregoing difficulties can all be overcome and a completely satisfactory inorganic nitrate composition pellet be prepared when we coat inorganic nitrate salts with a plasticized polyvinyl chloride composition, the inorganic nitrate salt being in the form of a symmetrical particle, preferably a sphere or a cylinder having a length approximately equal to its diameter, the particle having dimensions which provide a surface area of such magnitude that a coating averaging at least 3 mils in thickness can be provided by a quantity of coating material representing less than 10% by weight of the weight of the coated pellet.

By the term inorganic nitrate salt, we refer primarily to the ammonium and alkali metal salts of nitric acid inasmuch as they are of greatest importance from an explosive viewpoint because they are readily available at sufiiciently low prices. However, the nature of the invention is such that it is obviously applicable to the other inorganic nitrate salts, such as the alkaline earth metal salts, for example, some of which have been used in special purpose explosives.

By the term plasticized polyvinyl chloride we refer to the composition having the formula (l-I CCI-lCl) in admixture With high-molecular-weight fatty acid esters or other plasticizer. The polyvinyl chloride composition is commercially available in powdered form.

The pellet of this invention can be prepared by a number of coating procedures. We have found that a satis- [factory procedure comprises dissolving the polyvinyl chloride and a plasticizer, for example, dioctyl phth-alate, in a volatile solvent, for example, tetrahydrofuran, to form a sprayable liquid, introducing the symmetrical particles of the inorganic nitrate salt into a heated tumbling barrel, spraying the polyvinyl chloride solution slowly onto the tumbling particles, and continuing the tumbling until the particles are uniformly coated and the solvent has evaporated.

In order to fully describe our invention, reference is now made to the following examples. It will be understood that the examples are illustrative only, the invention not being limited to the specific proportions and procedures described.

Example 1 Granular ammonium nitrate 20% on 35 mesh, 15- 35% through 100 mesh) was formed on a 2 ton tableting press into a pellet with a density of 1.4 gram/ cc. a weight of 0.900 gram and a cylindrical body inch in diameter with each cylinder end capped with a hemisphere such that the thickness of the pellet was also inch.

To 2610 parts of tetrahydrofuran was added 300 parts polyvinyl chloride and 90 parts dioctyl phthalate, the ingredients were agitated thoroughly for approximately 30 minutes, and refluxed for 30 minutes.

The pellets (2000 parts by weight) were coated by spraying 615 parts of the solution (equivalent to parts by weight of solids) as they tumbled in a rotating barrel. The spray was applied by air atomization and a warm air stream (29 cubic feet/ minute at 52 C.) was directed through the bed of tumbling pellets to remove the solvent. The time required for coating and drying was approximately 1 hour. The coating may be sprayed continuously with continuous drying, intermittently with continuous drying, or intermittently with intermittent drying.

The weight of the pellets falling over one another was sumcient to smooth the plasticized polyvinyl chloride on in a thin continuous coat. The high vapor pressure of the solvent together With the warm air stream directed through the bed of pellets removed the solvent very rapidly and no sticking problem was encountered. This coating was 4% by weight of the coated pellet and has an average thickness of 7.5 mils.

The coated pellets were divided into several quantities. One sample of 300 parts Was placed under water (20 C.) in a pressure bomb at 20 p.s.i. ga. for 24 hours, after which the sample was dried and weighed to determine the Weight percent of undissolved material. A second sample of 300 grams was subjected to heat cycle storage days Where the pellets were exposed alternately to ambient temperature for 12 hours and 49 C. for 12 hours) and was then placed under water in a pressure bomb at p.s.i. ga. for 24 hours, after which the sample was dried and weighed to determine the weight percent of undissolved material. The results of the two tests were 94.7% and 84.1%, respectively.

Example 2 Example 3 In this run the pellets and the coating were prepared in the same manner as cited in Example 1. The pellets were coated in a rotating pan tilted at a angle and rotating at 20 r.p.m. The pellets, 2000 parts by weight, were coated by spraying 1000 parts of plasticized polyvinyl chloride solution (equivalent to 130 parts solids) on the pellets as they tumbled in the rotating pan. Concurrent with the spraying, 130 parts of powdered ferrosilicon was dusted onto the pellets. A warm air stream (70 C.) was directed into the rotating pan to remove the solvent. The time required for coating and drying was approximately 1 hour. The plasticized polyvinyl chloride coating was 6% by weight of the coated pellet and the coating', including the ferrosilicon, had an average thickness of 15.0 mils. Water resistance tests as previously described gave the following results with these pellets before and after heat cycle storage: 97.0% and 86.7%, respectively.

Example 4 In this run the pellets were formed in the same manner as in Example 1, and the solution for spraying the pellets was also prepared as in Example 1 except that the coating composition was as follows: 240 parts of polyvinyl chloride, 75 parts dioctyl phthalate, parts atomized modified fatty acid ester (Acrawax C), and 2625 parts tetrahydrofuran. The coating was applied using the procedure described in Example 3, except that no ferrosilicon was added. The coating represented 6% by weight of the coated pellet and had an average thickness of 6.5 mils. Water resistance tests previously described gave the following results with these pellets before and after heat cycle storage: 93.5% and 78.4%, respectively.

Example 5 The coated pellets were prepared in the same manner as described in Example 4, except that the coating composition was 240 parts of polyvinyl chloride, 75 parts of dioctyl phthalate, 90 parts of castor oil, and 2595 parts of tetrahydrofuran. This coating was 6.0% by weight of the coated pellet and had an average thickness of 7.8 mils. Results of water resistance tests before and after heat cycle were 98.0% and 80.0%, respectively.

4 Example 6 In this run sodium nitrate 2% on 10 mesh, 14% on 20 mesh, 1540% through 100 mesh) was formed into pellets (each weighing 1.100 grams with a density of 1.7 grams/ cc.) in the same manner as the pellets in Example 1. The solution for spraying these pellets was also prepared as in Example 1 except that the coating composition was 183 parts of polyvinyl chloride, 72 parts of dioctyl phthalate, 2745 parts of tetrahydrofuran. The coating was applied using the precedure described in Example 3. The plasticized polyvinyl chloride coating was 6% by weight of the coated pellet and had an average thickness of 5.0 mils. Results of water resistance tests before and after heat cycle were 82.1% and 97.9%, respectively.

Example 7 In this run 2400 parts ammonium nitrate, 2400 parts sodium nitrate, 300 parts corn starch, and 300 parts ferrosilicon were heated in a mixer to 60 C. at which time 300 parts of molten DNT was added. The temperature was increased to C.; 300 parts of sodium thiosulfate was added, and mixing was continued for approximately 15 minutes. The composition became plastic as the Water of crystallization, released by the sodium thiosulfate, gelatinized the corn starch and dissolved the nitrates. After mixing for the prescribed time, the composition was grained out in the mixer by rapid cooling.

The grained composition was passed through a IO-mesh and then a 20-mesh screen in order to provide a uniform free-running material which was fed as in Example 1 to a 2-ton tableting press. The pellet produced has a density of 1.9 grams/cc, a weight of 0.600 gram, and a cylinidrical body inch in diameter with each cylinder end capped with a hemisphere such that the thickness of the pellet was 7 inch.

The solution for spraying these pellets was made as in Example 1 except that the coating composition was as follows: 240 parts polyvinyl chloride, 75 parts dioctyl phthalate, 30 parts atomized modified fatty acid ester (Acrawax C), and 2655 parts tetrahydrofuran. The coating was applied using the procedure described in Example 3, except that on 2000 parts of pellets, 870 parts of solution (equivalent to parts solids) was sprayed and 100 parts ferrosilicon was dusted. This coating was 5% by weight and had an average thickness of 12.5 mils. Results of water resistance tests before and after heat cycle were 100% and 98.9%, respectively.

The foregoing examples illustrate not only the excellent water-resistance of the pellets of this invention, but also the capacity for modification to provide better explosive compositions. For example, the inclusion of ferro silicon provides greater density to the pellet and adds to the explosive strength. The incorporation of blends of inorganic nitrates with or without combustibles permit full adjustment of the oxygen balance of the pellet.

As previously indicated, the amount of coating used will be such as to provide an average thickness of coating of at least 3 mil but Will not exceed 10% by weight of the finished pellet. An amount of coating sufiicient to provide an average of at least 3 mil thickness over the entire surface of each pellet represents a minimum at which a substantial percentage of the coated pellets will have satisfactory water resistance. In any coating process, some irregularities are inevitable, and some pellets will have areas in which the coating thickness is less than 3 mils. If the coating thickness is substantially less, the

pellet will not withstand the immersion tests. As the average thickness of the coating is increased, the percentage of pellets failing the immersion tests decreases. Therefore, we preferably use an amount of coating which will provide an average coating thickness of over 3 mils, for example from 7 to 15 mils.

In counter-balance to the desire for a heavy coating in order to insure a high percentage of waterproofed pellets in the requirement that for satisfactory functioning of the pellet as an explosive, the quantity of coating must not substantially exceed the stoichiometric amount required to consume the excess oxygen available from the nitrate salts. A negative oxygen balance is bad because of fume and reduced explosive strength. In addition, too much coating material will have a desensitizing eiiect and may cause the pellets to fail to become initiated even in large diameter and primed by a strong primer. For example, about 87 parts of pure ammonium nitrate are required to provide the oxygen needed to fully oxidize about 13 parts of pure polyvinyl chloride. The oxygen requirements of the plasticizers usually are somewhat greater. Also, the inclusion of other fuels, either in the pellet or in the coating, is desirable for control of sensitivity and density. Accordingly, the amount of plasticized polyvinyl chloride used as the coating material should not exceed of the weight of the finished pellet.

In order to'achieve a plasticized polyvinyl coating having an average thickness of over 3 mils and still remain within the limit of not more than 10% by weight of the coated pellet automatically requires that the pellet have such dimensions that the proportion of coating composition will be adequate to provide the minimum average thickness. Assuming the optimum surface area to volume configuration, i.e., a perfect sphere, for the uncoated pellet, and assuming further that in the case where the thickness of the coating is relatively insignificant compared to the radius of the sphere (i.e., that the surface area of the coating is equal to that of the pellet), a minimum size pellet can be calculated from the equation:

W,, J, 7' we dp X 3t wherein r represents the radius of the pellet, W represents the weight of the pellet, W represents the weight of the coating composition, d represents the density of the pellet, d represents the density of the coating material, and t represents the thickness of the coating material.

In addition to the requirement that the pellet have sufiiciently large dimensions for minimum allowable coating with the maximum permissible amount of coating composition, we have found that pellet size has a direct bearing on the uniformity of coating obtainable by ordinary coating methods. Small pellets tend to agglomerate during the coating operation, and much of the coating composition is collected in the spaces between the particles. Further, when the agglomerates are broken 0.25 inch. Obviously, the maximum size is governed by the use to which the pellet is to be put rather than by any limitations imposed by the coating requirements.

The plasticized polyvinyl chloride composition has 5 sufficient flexibility to withstand the stresses produced by the change in density of an ammonium nitrate Pellet during transition. When heated through 32 C., ammonium nitrate changes its form and density to such an extent that a pellet increases approximately 2% in diameter. Such increase is suficient to crack any rigid coating. The flexibility and strength of the present coating was fully tested when some of the coated pellets described in Example 1 were placed on the floor and stomped on until they were crushed fiat. Upon immersion in Water, approximately the normal percentage remained satisfactory.

The invention has been fully described in the foregoing. We intend, therefore, to be limited only by the following claims.

We claim:

1. A Waterproof insensitive explosive pellet consisting essentially of a symmetrical core of at least one inorganic nitrate surrounded by a continuous coating of a plasticized polyvinyl chloride composition, said core being of such dimensions that an amount of coating of less than 10% by weight of said pellet will be sufiicient to provide an average coating thickness of at least 3 mils.

2. A pellet as claimed in claim 1, wherein said core is substantially spherical and has a diameter of at least 0.25 inch.

30 3. A pellet as claimed in claim 1, wherein said core is substantially cylindrical and has a diameter and length of at least 0.25 inch.

4. A pellet as claimed in claim 1, wherein said core comprises ammonium nitrate.

5. A pellet as claimed in claim 1, wherein said core comprises sodium nitrate.

6. A pellet as claimed in claim 1, wherein said core comprises a blend of ammonium nitrate and potassium nitrate.

References (lited in the file of this patent UNITED STATES PATENTS 1,929,453 Semen Oct. 10, 1933 2,155,499 Lawson Apr. 25, 1939 2,171,379 Wahl Aug. 29, 1939 2,932,251 Hamilton Apr. 12, 1960 2,973,255 Eiszner Feb. 28, 1961 OTHER REFERENCES Bebie: Explosives, Military Pyrotechnics, and Chemical Warfare Agents, 1943, page 25.

Gregory: Uses and Applications of Chemicals and Related Materials, vol. II, page 263. 

1. A WATER PROOF INSENSITIVE EXPLOSIVE PELLET CONSISTING ESSENTIALLY OF A SYMMETRICAL CORE OF AT LEAST ONE INORGANIC NITRATE SURROUNDED BY A CONTINOUS COATING OF A PLASTICIZED POLYVINYL CHLORIDE COMPOSITION SAID CORE BEING OF AUCH DIMENSIONS THAT AN AMOUNT OF COATING OF LESS THAN 10% BY WEIGHT OF SAID PELLET WILL BE SUFFICIENT TO PROVIDE AN AVERAGE COATING THICKNESS OF AT LEAST 3 MILS. 